Nitric oxide synthases (NOS), the enzymes responsible for nitric oxide (NO) production from the substrate L-arginine, are also NADPH oxidases. In cell-free systems, some of these enzymes have been shown to produce reactive oxygen species such as superoxide. In this investigation, we transfected monoblastoid U937 cells with human endothelial NOS (eNOS) and found that TNFa production was increased, but that this effect was not related to NO production. Further work has found that eNOS upregulates TNEa by producing a reactive oxygen species (ROS) (Abstract, Invest Med, 1998; manuscript in preparation). Future work will focus on the mechanisms that regulate eNOS to produce either NO or ROS, and on how eNOS modulates the inflammatory response of endothelial cells.